9pfs: local: fix fchmodat_nofollow() limitations

This function has to ensure it doesn't follow a symlink that could be used
to escape the virtfs directory. This could be easily achieved if fchmodat()
on linux honored the AT_SYMLINK_NOFOLLOW flag as described in POSIX, but
it doesn't. There was a tentative to implement a new fchmodat2() syscall
with the correct semantics:

https://patchwork.kernel.org/patch/9596301/

but it didn't gain much momentum. Also it was suggested to look at an O_PATH
based solution in the first place.

The current implementation covers most use-cases, but it notably fails if:
- the target path has access rights equal to 0000 (openat() returns EPERM),
  => once you've done chmod(0000) on a file, you can never chmod() again
- the target path is UNIX domain socket (openat() returns ENXIO)
  => bind() of UNIX domain sockets fails if the file is on 9pfs

The solution is to use O_PATH: openat() now succeeds in both cases, and we
can ensure the path isn't a symlink with fstat(). The associated entry in
"/proc/self/fd" can hence be safely passed to the regular chmod() syscall.

The previous behavior is kept for older systems that don't have O_PATH.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Zhi Yong Wu <zhiyong.wu@ucloud.cn>
Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

1 files changed, 35 insertions, 7 deletions

diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
index 6e478f4765..efb0b79a74 100644
--- a/hw/9pfs/9p-local.c
+++ b/hw/9pfs/9p-local.c
@@ -333,17 +333,27 @@ update_map_file:
 
 static int fchmodat_nofollow(int dirfd, const char *name, mode_t mode)
 {
+    struct stat stbuf;
     int fd, ret;
 
     /* FIXME: this should be handled with fchmodat(AT_SYMLINK_NOFOLLOW).
-     * Unfortunately, the linux kernel doesn't implement it yet. As an
-     * alternative, let's open the file and use fchmod() instead. This
-     * may fail depending on the permissions of the file, but it is the
-     * best we can do to avoid TOCTTOU. We first try to open read-only
-     * in case name points to a directory. If that fails, we try write-only
-     * in case name doesn't point to a directory.
+     * Unfortunately, the linux kernel doesn't implement it yet.
      */
-    fd = openat_file(dirfd, name, O_RDONLY, 0);
+
+     /* First, we clear non-racing symlinks out of the way. */
+    if (fstatat(dirfd, name, &stbuf, AT_SYMLINK_NOFOLLOW)) {
+        return -1;
+    }
+    if (S_ISLNK(stbuf.st_mode)) {
+        errno = ELOOP;
+        return -1;
+    }
+
+    /* Access modes are ignored when O_PATH is supported. We try O_RDONLY and
+     * O_WRONLY for old-systems that don't support O_PATH.
+     */
+    fd = openat_file(dirfd, name, O_RDONLY | O_PATH_9P_UTIL, 0);
+#if O_PATH_9P_UTIL == 0
     if (fd == -1) {
         /* In case the file is writable-only and isn't a directory. */
         if (errno == EACCES) {
@@ -357,6 +367,24 @@ static int fchmodat_nofollow(int dirfd, const char *name, mode_t mode)
         return -1;
     }
     ret = fchmod(fd, mode);
+#else
+    if (fd == -1) {
+        return -1;
+    }
+
+    /* Now we handle racing symlinks. */
+    ret = fstat(fd, &stbuf);
+    if (!ret) {
+        if (S_ISLNK(stbuf.st_mode)) {
+            errno = ELOOP;
+            ret = -1;
+        } else {
+            char *proc_path = g_strdup_printf("/proc/self/fd/%d", fd);
+            ret = chmod(proc_path, mode);
+            g_free(proc_path);
+        }
+    }
+#endif
     close_preserve_errno(fd);
     return ret;
 }